Overview of RF Switch
Peregrine Semiconductor, San Diego, CA
Drew Fischer and Ruan Lourens
National Instruments, Austin, TX
Modern RF instrumentation is heavily dependent on switch technology. As both
power consumption and space requirements shrink, selecting the appropriate
switch solution becomes increasingly important. This article provides an
overview of RF switches and considerations for selection based on function within
the RF instrument.
OVERVIEW OF TECHNOLOGIES
Solid State Switches
Solid state switches can be divided into two primary categories: diode and field-effect ransistor (FET). PIN diode switches are
generally realized using discrete implementations and are known for higher power handling
and fast switching speed, but the fairly complex
biasing schemes and high levels of DC power
required are significant disadvantages, especially for battery-operated instrumentation.
PIN diodes require a forward current through
the device to establish a low series resistance.
This direct injection of DC current into the RF
channel limits low-frequency operation.
FET-based switches are more commonly
found in integrated solutions because the
switching behavior is voltage-dependent. The
control voltage applied to the gate of the FET
“switches” the channel from a low-resistance
“on” state to a high-resistance, capacitive “off”
state. Three-terminal FET devices feature
minimal DC power consumption and separate
gate control of the channel. The high impedance of the gate supports a broadband response, but the FET is still frequency limited
by finite capacitance between the RF channel
and the gate terminal, as well as channel capacitance when the device is in the “off” state.
There are two distinct FET types commonly
used in industry today: MESFETs and MOSFETs. MESFETs are fabricated using gallium
arsenide (GaAs) and gallium nitride (GaN)
processes, while MOSFET devices are commonly silicon-based. A primary distinction between the two types is the asymmetric behavior
of the MESFET gate to applied voltage.
Electromechanical and MEMS Switches
Electromechanical (EM) switches use a different mechanism than their solid state counterparts. In an EM switch, a metal contact is
actuated to make or break the connection. This
technology offers the advantages of low insertion loss, high isolation, and high linearity.1
Emerging microelectromechanical systems
(MEMS) switch technology attempts to deliver
the advantages of traditional EM switches, but
in a small form factor. MEMS switches employ
micro-miniaturized mechanical contacts controlled by electrostatic forces to make RF connections.1, 2
No single switch solution can satisfy all of
the various requirements for each application.
Frequency range, power handling, linearity,
switching speed, insertion loss, isolation, video
feedthrough, power consumption, and reli-